Optical permitting record/playback from both sides of an optical disk

ABSTRACT

An optical assembly of an optical recording/reproducing apparatus for focusing an optical beam produced by a light source on a recording medium and for directing the optical beam reflected back from the recording medium to detectors comprises a first optical system including a first objective lens for focusing an optical beam on a first recording surface at a first side of a recording medium, a second optical system including a second objective lens for focusing the optical beam on a second recording surface at a second side of the recording medium, and an optical path switching means switched between a first and second states for selectively establishing an optical path connecting an optical processing unit including the light source and detectors to either one of the first and second optical subsystems.

BACKGROUND OF THE INVENTION

The present invention generally relates to optical recording/reproducingsystems, and in particular to an optical assembly of an opticalrecording/reproducing apparatus recording and/or reproducing aninformation signal on and from both sides of a disk-shaped informationrecording medium by means of an optical beam for focusing the opticalbeam on the recording medium.

Generally, conventional optical disk recording/reproducing apparatusrecords and/or reproduces an information signal on and from one side ofa disk-shaped recording medium (referred to hereinafter as a disk) bymeans of an optical recording/reproducing system. Such an opticalrecording/reproducing system comprises a light source, an opticalassembly for focusing an optical beam, focusing and tracking mechanismsfor controlling the optical beam, and a light detecting element. Theoptical recording/reproducing system is sometimes referred to as anoptical head and radiates an optical beam on the disk and receives theoptical beam reflected back from the disk. In such a disk, it isnaturally desirable that the both sides of the disk can be used forstorage of the information signal so that the recording capacity per onedisk is increased.

In order to enable the recording and reproducing of the informationsignal on and from the both sides of the disk, one may use a well knownautomatic turn-over mechanism which turns the disk upside down whileusing a single optical head for focusing the optical beam on the lowerside of the disk, for example. However, such automatic turn-overmechanism is complex and occupies a large space not only due to themechanism itself but also due to the movement of the disk at the time ofturn-over. Therefore, the optical disk recording and reproducingapparatus using such automatic turn-over mechanism inevitably becomesbulky and complex.

Also, there is a known optical disk recording apparatus for recording anoptical information signal on both sides of an optical disk according tothe Laid-open Japanese Patent Application No. 54-35707, in which a pairof optical recording systems are provided on both sides of the disk forrecording the information signal on the both sides of the disk. In thisconventional recording apparatus, two bulky and heavy optical recordingsystems have to be used and therefore the apparatus inevitably becomeslarge and heavy. Further, the optical recording and reproducingapparatus using the construction as disclosed by the Laid-open JapanesePatent Application No. 54-35707 has a problem that a large access timeis required for the optical recording/reproducing system to move to adesired track position due to the inertia of the opticalrecording/reproducing system.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anoptical recording/reproducing system in which the aforementionedproblems are eliminated.

Another and more specific object of the present invention is to providean optical assembly of an optical recording/reproducing apparatus forselectively focusing an optical beam on both sides of an opticalrecording medium without turning the side of the recording medium.

Another object of the present invention is to provide an opticalassembly for focusing an optical beam on both sides of a disk-shapedoptical information recording medium in which a first optical beam isfocused on a first recording surface at an upper side of the diskthrough a first optical system including a first objective lens locatedabove the level of the disk for recording and/or reproducing aninformation signal on and from the upper side of the disk, a secondoptical beam is focused on a second recording surface at a lower side ofthe disk through a second optical system including a second objectivelens located below the level of the disk for recording and/orreproducing the information signal on and from the lower side of thedisk, the optical beam is exited from and returned to an opticalprocessing unit which comprises a light source and detecting means, saidoptical processing unit produces the optical beam for recording andreproducing and further produces a tracking error signal, a focusingerror signal and a reproduced high frequency signal corresponding to theinformation signal recorded on the disk on the basis of the reflectedoptical beam reflected back from the optical disk thereto and detectedby the detecting means, whereby the optical path of the optical beamexited from the optical processing unit is switched between the firstand second optical systems by switching means interposed in the opticalpath of the optical beam from the optical processing unit.

According to the present invention, the optical disk recording andreproducing apparatus can be made compact and the access time for adesired track is reduced. More specifically, the optical assembly of thepresent invention cooperates with only one optical beam processing unitfor recording and reproducing the information signal on and from bothsides of the disk. Thus, the number of bulky and heavy opticalprocessing unit can be reduced and the size of the apparatus can besignificantly reduced. Further, the access time is reduced as a resultof the use of the first and second optical systems which are relativelylight in weight. Thus, the efficiency in the operation of the apparatusis improved. Furthermore, the cost of the optical disk is reduced, as asignificant number of expensive optical elements used in the opticalprocessing unit are saved.

Still other objects and further features of the present invention willbecome apparent from the following detailed description on the preferredembodiments when read in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an embodiment of the opticalrecording/reproducing system of the present invention as applied to amagneto-optical disk recording and reproducing apparatus; and

FIG. 2 is a schematic circuit diagram showing a part of the opticalrecording/reproducing system of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows an embodiment of the optical recording/reproducing systemof the present invention as applied to a magneto-optical disk recordingand reproducing apparatus. Referring to the drawing, an opticalrecording/reproducing system 1 referred to hereinafter as an opticalhead comprises a first moving part 3 which is located below the level ofa magneto-optical disk 2 referred to hereinafter as a disk and controlsthe focusing and tracking of an optical beam irradiated on a firstrecording surface 2a at the lower side of the disk 2. At the time ofrecording, the optical beam heats the magnetic material on the recordingsurface and the magnetic material is magnetized in a direction of amagnetic field applied externally on the disk upon cooling of themagnetic material. At the time of reproduction, the plane ofpolarization of the optical beam is rotated when the optical beamincident to the recording surface is reflected at the recording surface2a. The direction and the amount of the rotation depends on thedirection of the magnetization of the magnetic material on the recordingsurface. Similarly, a second moving part 4 having an identicalconstruction to the first moving part 3 is located above the disk 2 andthe disk 2 is recorded with the information signal on a second recordingsurface 2b at the upper side of the disk 2. Thus, the information signalis recorded on and reproduced from both sides of the disk 2 withoutturning over the side of the disk.

The first and second moving parts 3 and 4 respectively comprise firstand second optical systems including first and second objective lenses3a and 4a which face to the first and second recording surfaces 2a and2b (illustrated by a broken line in FIG. 1) of the disk 2 and mirrors 3band 4b, and focusing actuators 3c and 4c for driving the lenses 3a and4a upwards and downwards. Further, the first and second moving parts 3and 4 are moved together along the radial direction of the disk by knowntracking actuators 5 and 6 using a linear motor and maintain a propertracking of the optical beam. The focusing actuators 3c and 4c are knownelectromagnetic actuators having a similar construction to the voicecoil assembly of a moving coil speaker and comprise a magnet and asolenoid. Thus, the focusing actuator maintains a proper focusing of theoptical beam on the recording surface of the disk by moving theobjective lens to and from the disk 2 responsive to a driving currentflowing through the solenoid. As will be described in detail later, themagnet and solenoid of the focusing actuators 3c and 4c are also used toproduce a magnetic flux which is used at the time of recording.

The magneto-optical disk recording and reproducing apparatus in FIG. 1further comprises an optical processing unit 7 which is fixed on a frameof the apparatus. The slashed portion below the unit 7 in the drawingdesignates the frame of the apparatus and indicates that the unit 7 isfixed to the frame of the apparatus. The unit 7 comprises a light source8 such as a laser diode for producing the optical beam, a trackingdetector 9 such as a PIN diode for producing a tracking error signalindicating the deviation of the optical beam from the proper tracking, afocusing detector 10 similar to the detector 9 for producing a focusingerror signal indicating the deviation of the optical beam from theproper focusing, and an information signal reproducing circuit 11 forreproducing the information signal in the form of a high frequencysignal (RF signal).

Further, there is provided a rotatable mirror 12 in an optical pathextending from the unit 7 to the first and second moving parts 3 and 4such that the mirror 12 is rotatable between first and second states.The mirror 12 is rotated by a motor 16 as will be described. In thefirst state, the mirror 12 moves out from the optical path and theoptical beam from the unit 7 is directed to the first recording surface2a after passing through the mirror 3b and the first lens 3a of thefirst moving part 3. In the second state, the mirror 12 interrupts theoptical path from the unit 7 to the first moving part 3 and the opticalbeam is reflected by the mirror 12 now in an interrupting position so asto hit another mirror 33, where the beam is further directed to thesecond recording surface 2b of the optical disk 2 after passing throughthe mirror 4b and the lens 4a of the second moving part 4.

As described heretofore, the optical head of the present invention usesa single and fixed optical processing unit 7 for radiating and receivingthe optical beam to and from the first and second optical systems on themoving parts 3 and 4, and the recording surface of the disk 2 can beselected simply by rotating the mirror 12 between the first and secondstates. Thus, a saving in the space conventionally occupied by theautomatic turn-over mechanism as well as by the relatively bulky opticalprocessing unit 7 is obtained and the optical disk recording andreproducing system can be made significantly compact.

Next, a description of operation of the recording and reproduction ofthe apparatus will be given in relation to further descriptions on thedetails of the structure of the optical head.

(1) Recording on the lower side of the disk

When recording an information signal on the first recording surface 2aat the lower side of the disk 2, the user provides a switching signalspecifying the lower side of the disk from an input terminal 13 to amotor drive circuit 14. The control signal may be a low level signal ora signal having a negative polarity, for example. Responsive to thecontrol signal, the motor drive circuit 14 which may be a linearamplifier produces an output signal which drives the motor 16 in acounter clockwise direction. Responsive to the driving of the motor 16,the mirror 12 is rotated in the counter clockwise direction and assumesthe first state indicated by a broken line in which the mirror 12 abutsto a first stop surface 17a of a stopper 17 and the motor 16 is stopped.In this state, the mirror 12 is located outside of the optical pathextending from the unit 7 to the first moving part 3 indicated by A inthe drawing. The mechanism to stop the motor 16 responsive to theabutting of the mirror 12 to the stop surface 17a may be any known suchmechanism and further description will be omitted.

Further, the same control signal is supplied to a switch drive circuit15 which activates a switch 18 which in turn switches the connectionbetween a bias drive circuit 19, focusing actuator drive circuit 20, andthe solenoids of the focusing actuators 3c and 4c as will be described.Referring to FIG. 2, the focusing error signal produced by the detector10 is supplied to a FOCUSING ERROR IN terminal of the driving circuit 20which produces a driving current of the solenoid of the focusingactuator 3c, 4c. The driving current is supplied from an output terminaland returns to another output terminal of the circuit 20. The focusingactuators 3c and 4c are, as previously described, of the known type inwhich the solenoid is fitted around a magnet core in a movable manneralong the axial direction of the magnet core as in the case of the voicecoil assembly of a moving coil speaker. The lenses 3a and 4a are carriedunitary with the respective solenoids. The switch 18 comprises a movingcontact 18a to be selectively connected to either one of contacts 19cand 20c, a moving contact 18b to be selectively connected to either oneof contacts 19d and 20d, a moving contact 18c to be selectivelyconnected to either one of contacts 19a and 20a, and a moving contact18d to be selectively connected to either one of contacts 19b and 20b.The bias drive circuit 19 has a similar construction as the focusingactuator drive circuit 20 in which a predetermined bias signal from arheostat 40 connected to a source voltage Vcc is supplied through aswitch 41 which is closed at the time of recording, and a predeterminedlevel of bias current is outputted from one of the output terminalsresponsive to the input bias signal and returns to the other outputterminal. The contacts 20a and 20c are connected to one of the outputterminals of the focusing actuator drive circuit 20 and the contacts 20band 20d are connected to the other output terminals of the circuit 20.Further, the contacts 19a and 19c are connected to one of the outputterminals of the bias drive circuit 19 and the contacts 19b and 19d areconnected to the other of the output terminals of the circuit 19.

Responsive to the control signal to the input terminal 13 specifying thelower side of the disk, the moving contacts 18a and 18b make contactwith the terminals 20c and 20d which are, as previously described,connected to the focusing actuator drive circuit 20. At the same time,the moving contacts 18c and 18d make contact with the terminals 19a and19b which are connected to the bias drive circuit 19. Thus, the outputdriving current from the focusing actuator drive circuit 20 flowsthrough the solenoid of the first actuator 3c located below the level ofthe disk 2. Thus, the solenoid is moved responsive to the focusing errorsignal and the distance between the lens 3a and the first recordingsurface 2a is adjusted. Further, the predetermined level of the biascurrent from the bias drive circuit 19 flows through the solenoid of thefocusing actuator 4c located above the disk 2 responsive to the closureof the switch 41 at the time of recording and applies a first magneticfield to the disk 2 at the portion on which the recording is to be madein a first direction as illustrated. This magnetic field is used forrecording as will be described.

Responsive to the activation of the recording mode by the user whichincludes the closing of the switch 41 as aforementioned, the laser diode8 is energized by the driving signal modulated with the informationsignal to be recorded at an input terminal 8a and a relatively intenseoptical beam modulated with the information signal is produced by thelaser diode 8. As shown in FIG. 1, this optical beam is outputted fromthe optical processing unit 7 in a form of a parallel beam after passingthrough a collimator lens 21, a grating 22, and beam splitters 23 and24. In the recording and reproduction of the lower side of the disk, themirror 12 is rotated to a position indicated by the broken line asdescribed previously. In other words, the mirror 12 is outside of theoptical path A. Therefore, the optical beam from the unit 7 directlyenters the moving part 3 at the lower side of the disk 2 In the movingpart 3, the optical beam is reflected by the mirror 3b and focused onthe first recording surface 2a of the disk 2 by the objective lens 3a.The optical beam reflected back from the recording surface 2a isreturned to the unit 7 after passing through the lens 3b and reflectionat the mirror 3b.

This reflected optical beam returned to the unit 7 is passed through thebeam splitter 24 and is then reflected by the beam splitter 23. Theoptical beam is then passed through another beam splitter 25 and acylindrical lens 26 and reaches the focusing detector 10. The focusingdetector 10 is a known detector having a quadrant imaging field forreceiving the optical beam and produces the focusing error signalresponsive to the difference between a sum of the intensity of lightreceived at the first and third quadrant and a sum of the intensity ofthe light received at the second and fourth quadrant. This differencerepresents the error in the distance between the recording surface 2aand the objective lens 3a as compared to the proper focal length. Theprinciple of the quadrant focusing detector is well known and furtherdescription thereof will be omitted.

The focusing error signal thus obtained is supplied to the focusingactuator drive circuit 20 and the circuit 20 produces the output drivecurrent which flows through the solenoid of the first focusing actuator3c as already described. As a result, the objective lens 3a is moved toand from the disk 2 and assumes a position in which the distance betweenthe lens and the recording surface 2a is substantially equal to thefocal length of the lens.

A part of the light reflected back from the disk 2 is reflected by thebeam splitter 25 and directed to the tracking detector 9. This trackingdetector 9 is also a known quadrant type detector having a quadrantimaging field. In such a detector, the spot of optical beam formed onthe quadrant imaging field shifts to one or the other direction when theoptical beam is offset and the proper tracking is lost. Thus, such adetector detects the tracking error as a difference between a sum of theintensity of the light detected in the first and second quadrant imagingfield and a sum of the intensity of the light detected in the third andfourth quadrant imaging field. The principle of such tracking detectoris well known and further description thereof will be omitted. Thetracking err or signal obtained by the detector 9 is supplied to thetracking actuators 5 and 6 of the first and second moving parts 3a and4. As previously described, the actuators 5 and 6 may be a linearactuator using a linear motor and moves the moving parts 3 and 4simultaneously in the radial direction to and from the disk such thatthe optical beam from the laser diode 8 falls exactly on a predeterminedgroove which forms the track on the disk. For this purpose, one at thefirst and second tracking actuators 5 and 6 may be driven by a singletracking error signal and the other tracking actuator may be connectedmechanically thereto. The line 100 in FIG. 1 indicates that the linearactuators 5 and 6 are driven simultaneously either electrically ormechanically. It should be noted that only the moving parts 3 and 4 aremoved in the optical head of the present invention. As the moving parts3 and 4 are small and light in weight, the time required for the opticalbeam to reach to the desired track is significantly reduced.

In the recording mode, the magnetic material on the first recordingsurface 2a is heated to a temperature above the Curie point and iscooled off subsequently responsive to the removal of the optical beam.Thereby, the magnetic material is magnetized in the direction of themagnetic field produced by the solenoid of the second focusing actuator4c. The intensity of the magnetic field may be optimized by adjustingthe rheostat 40. The principle of such magneto-optical recording and thecontrol of the optical beam therefor is well known and no furtherdescription will be given thereof.

(2) Reproduction from the lower side of the disk

Next, description will be given for the case in which the informationsignal is recorded on the first recording surface 2a of the lower sideof the disk 2. As the focusing control and tracking control areidentical to the case of the recording on the lower side of the disk,the description thereof will be omitted.

The optical beam reflected at the recording surface 2a has a plane ofpolarization which is rotated by a predetermined angle responsive to thedirection of the magnetization of the recording surface 2a. Thus, whenthe direction of magnetization is in the upward direction, the plane ofpolarization of the reflected beam rotates to a first direction whilethe plane of polarization rotates to an opposite direction when thedirection of magnetization of the recording surface is in the downwarddirection. As a result, the reflected optical beam from the recordingsurface 2a has different plane of polarization depending on thedirection of the magnetization of the recording surface of the disk. Forexample, the reflected beam may have a plane of polarization rotated ina clockwise direction when the recording surface is magnetized in theupward direction and may have the plane rotated in a counter clockwisedirection.

The reflected optical beam is then passed through the objective lens 3aand returns to the optical processing unit 7 after reflection at themirror 3b. The optical beam thus reflected from the disk 2 is thenpassed through a half wave plate 27 where the plane of polarization isrotated by 45°. The optical beam is then passed through a beam splitter28 which is a polarization beam splitter designed to pass preferentiallyan optical beam having a first plane of polarization and to reflect anoptical beam having a second plane of polarization which isperpendicular to the first plane of polarization. The optical beampassed through the beam splitter 28 is then directed to a detector 31after reflection at a mirror 30 while the optical beam reflected by thebeam splitter 28 is received by a detector 29. The detectors 31 and 29are respectively connected to a non-inverting input terminal and aninverting input terminal of a differential amplifier 32 which producesan output high frequency signal corresponding to the information signalrecorded on the recording surface 2a.

Referring to FIG. 1 again, the optical beam reflected by the recordingsurface 2a having the upward magnetizing direction and having the planeof polarization rotated to a first direction is preferentially passedthrough the beam splitter 28 and reaches the detector 31. On the otherhand, the reflection of the same optical beam at the beam splitter 28 isminimized and no substantial optical beam arrives at the detector 29.Thus, the detector 31 produces a high level output to be supplied to thenon-inverting input terminal of the amplifier 32 and the detector 29produces a low level output to be supplied to the inverting inputterminal of the detector 32. As a result, the detector 32 produces ahigh level output signal responsive to the upward magnetization of therecording surface 2a. When the direction of magnetization of therecording surface 2a is in the downward direction, the optical beam ispreferentially supplied to the detector 29 while no substantial opticalbeam is detected by the detector 31. Thus, a high level signal isapplied to the inverting input terminal of the amplifier 32 from thedetector 29 and a low level signal is applied to the non-inverting inputterminal of the amplifier 32. As a result, the differential amplifier 32produces a low level output responsive to the downward magnetization ofthe recording surface of the disk 2. It should be noted that the halfwave plate 27 used to rotate the plane of polarization of the reflectedoptical beam by 45° may be rotated around the center of the optical pathby a suitable amount such that the plane of polarization matches theoperation of the polarization beam splitter 28 and that the optical beamis properly passed or reflected by the beam splitter 28 depending on theplane of polarization. With this adjustment, the intensity of theoptical beam detected by the detector 31 at the time of upwardmagnetization of the recording surface and the intensity of the opticalbeam detected by the detector 29 at the time of downward magnetizationof the recording surface are set approximately equal.

(3) Recording on the upper side of the disk

In this recording mode, the user first selects the recording surface 2bof the disk 2 by providing the control signal specifying the secondrecording surface to the input terminal 13. The control signal may be ahigh level signal or signal having a positive polarity. Responsive tothe control signal, the motor drive circuit 14 and the switch drivecircuit 15 produce output signals to the motor 16 and to the switch 18.Responsive to the output signal from the motor drive circuit 14, themotor 16 is driven and the mirror 12 is rotated in a clockwisedirection. The mirror then abuts to a horizontal stop surface 17b of thestopper 17 as shown in the solid line and the motor 16 is stopped inthis state in which the mirror is located within the optical path A.Thus, the optical beam exited from the optical processing unit 7 isreflected by the mirror 12 and directed to the second moving part 4after reflection at the stationary mirror 33, wherein the optical beamis focused on the second recording surface 2b of the disk 2 aspreviously described.

At the same time as the rotation of the mirror 12, the switch drivecircuit 15 activates the switch 18 as illustrated by the solid line inFIG. 2. In this state, the moving contacts 18a and 18b make contact withthe terminals 19c and 19d of the bias drive circuit 19. Further, themoving contacts 18c and 18d make contact with the terminals 20a and 20bconnected to the focusing actuator drive circuit 20. Thereby, thesolenoid of the focusing actuator 4c is supplied with the drive currentfrom the focusing actuator drive circuit 20 responsive to the focusingerror signal.

Responsive to the activation of the recording mode, the optical beamproduced by the laser diode 8 of FIG. 1 is exited from the opticalprocessing unit 7 as previously described. As the mirror 12 is in thesecond state as indicated by the solid line, the optical beam from theunit 7 is reflected by the mirror 12 in the upward direction and entersthe second moving part 4 after reflection at the mirror 33. The opticalbeam is received by the mirror 4b and is focused on the second recordingsurface 2b after passing through the second lens 4a.

The optical beam reflected at the second recording surface 2b traces thesame optical path of the incident optical beam in the backward directionand returns to the optical processing unit 7. The reflected optical beamfrom the disk 2 then reaches the focusing detector 10 and the trackingdetector 9 as already explained with reference to the case of recordingon the lower side of the disk. The tracking error signal from thefocusing detector 10 is amplified by the focusing actuator drive circuit20 as already described and the output drive signal of the circuit 20flows through the solenoid of the second focusing actuator 4c.Responsive to the driving current, the focusing actuator 4c moves thesecond objective lens 4a to and from the disk 2 and the distance betweenthe lens 4a and the second recording surface 2b is optimized. Similarly,the tracking error signal form the tracking detector 9 is supplied tothe actuators 5 and 6 and the first and second actuators are movedtogether in the radial direction of the disk 2 in a unitary manner.

Responsive to the activation of the recording mode, the switch 41 at theinput of the bias drive circuit 19 is closed similarly to the case ofrecording on the lower side of the disk, and the bias drive circuit 19outputs the drive current which flows through the solenoid of thefocusing actuator 3c located below the disk 2. The leakage flux of thesolenoid provides a second magnetic field which is necessary at the timeof recording by the optical beam. The direction of the second magneticfield may be opposite to the direction of the first magnetic field.Then, the optical beam from the moving part 4 in the upper side of thedisk 2 heats the magnetic material on the recording surface 2b to atemperature which exceeds the Curie point, and the magnetic material ismagnetized in the direction of the magnetic field upon cooling due tothe removal of the optical beam.

(4) Reproduction from the upper side of the disk

Next, description will be given for the reproduction of the informationsignal recorded on the second recording surface 2b at the lower side ofthe disk 2. As the focusing and tracking control is made similar to thecase of the reproduction of the information signals from the firstrecording surface 2b on the upper side of the disk 2, the descriptionthereof will not be repeated here.

The optical beam focused on the recording surface 2b and reflectedtherefrom has a plane of polarization which is rotated responsive to thedirection of magnetization of the recording surface 2b. The reflectedoptical beam thus produced is returned back to the optical processingunit 7 after passing through the objective lens 4a and being reflectedby the mirror 4b, mirror 33 and the mirror 12. The optical beam is thenprocessed as already described in the unit 7 and the reproduced highfrequency signal corresponding to the information recorded on the secondrecording surface 2b is obtained from the differential amplifier 32.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention. For example, the present inventioncan also be applied to general type of optical disks other than themagneto-optical type disk as described. Further, the magnetic field usedfor the recording by the optical beam may also be obtained from thesolenoid of the focusing actuator itself during the use cf the solenoidfor the focusing control of the optical beam. Furthermore, the magneticfield produced by the solenoid of the focusing actuator in the oppositeside of the disk may be augmented or diminished responsive to theintensity of the magnetic flux from the solenoid of the focusingactuator located on the one side of the disk.

What is claimed is:
 1. An optical assembly of an opticalrecording/reproducing apparatus recording and/or reproducing aninformation signal on and from a recording surface of a disk-shapedinformation recording medium by means of an optical beam produced by anoptical processing unit including a light source and optical beamdetecting means, said optical assembly being supplied with the opticalbeam from the optical processing unit and focusing the optical beam onthe recording surface of the recording medium and directing the opticalbeam reflected back from the recording medium to the optical processingunit comprising:a first optical system located at a first side of therecording medium and supplied with the optical beam from the opticalprocessing unit for focusing the supplied optical beam on a firstrecording surface of the recording medium at the first side, said firstoptical system further receiving the optical beam reflected back fromthe first recording surface of the recording medium and outputting thereflected optical beam to the optical processing unit; a second opticalsystem located at a second side of the recording medium and suppliedwith the optical beam from the optical processing unit for focusing thesupplied optical beam on a second recording surface of the recordingmedium at the second side, said second optical system further receivingthe optical beam reflected back from the second recording surface of therecording medium and outputting the reflected optical beam to theoptical processing unit; and optical path switching means located in apath of the optical beam radiated from the optical processing unit, saidoptical path switching means being supplied with a control signal andselectively establishing an optical path which extends from the opticalprocessing unit to either one of the first and second optical systemsand passing the optical beam from the optical processing unitselectively to either one of the first and second optical systems andfor passing the optical beam reflected back from the first and secondoptical systems to the optical processing unit responsive to the stateof the control signal.
 2. An optical assembly as claimed in claim 1 inwhich said optical path switching means is operated between a first andsecond states responsive to the control signal and supplies the opticalbeam from the optical processing unit to said first optical system andsupplies the reflected optical beam from the first optical system to theoptical processing unit when the control signal is in a first electricalstate, said optical path switching means further supplies the opticalbeam from the optical processing unit to said second optical system andsupplies the reflected optical beam from the second optical system tothe optical processing unit when the control signal is in a secondoptical state.
 3. An optical assembly as claimed in claim 2 in whichsaid first optical system comprises a first objective lens for focusingthe optical beam and a first optical path means for guiding the opticalbeam supplied from the optical processing unit via the optical pathswitching means to the first objective lens, said first optical pathmeans further directing the optical beam reflected from the firstrecording surface and received by the first objective lens to theoptical processing unit via said optical path switching means, and saidsecond optical system comprises a second objective lens for focusing theoptical beam and a second optical path means for guiding the opticalbeam supplied from the optical processing unit via the optical pathswitching means to the second objective lens, said second optical pathmeans further directing the optical beam reflected from the secondrecording surface and received by the second objective lens to theoptical processing unit via said optical path switching means.
 4. Anoptical assembly as claimed in claim 3 in which said first and secondobjective lenses are carried by first and second electromagneticactuators in a movable manner in the direction to and from the first andsecond recording surfaces, respectively.
 5. An optical assembly asclaimed in claim 2 in which said optical path switching means comprisesa rotary mirror rotated between first and second positions responsive tothe first and second electrical states of the control signalrespectively, an electric motor connected to the rotary mirror forrotating the mirror between the first and second positions, and adriving circuit for driving the motor responsive to the control signalsuch that the mirror is rotated between the first and second positions.6. An optical assembly as claimed in claim 5 in which the rotary mirrorassumes a position outside of the path of the optical beam from theoptical processing unit when the rotary mirror is in the first position,and the rotary mirror assumes a position so as to reflect the opticalbeam from the optical processing unit to the second optical subsystemwhen the rotary mirror is in the second position.